Fluorescent tubes are classified roughly into hot-cathode tubes and cold-cathode tubes depending on the configuration of the electrodes thereof. The electrodes of a cold-cathode tube (also referred to as a CCFL) are formed of substances that emit numerous electrons through the application of high voltage. Namely, the electrodes do not include any filaments for emitting thermal electrons, unlike the case of the hot-cathode tubes. For this reason, the cold-cathode tubes are particularly advantageous over the hot-cathode tubes in terms of very small tube diameter, long life and low power consumption. Because of the advantages, the cold-cathode tubes are mainly used frequently for products strongly requested to be made thinner (or smaller in size) and lower in power consumption, such as the backlights of liquid crystal displays, the light sources of facsimiles and scanners.
The cold-cathode tubes have electrical characteristics of higher firing potential, smaller discharge current (referred to as tube current hereinafter) and higher impedance than the hot-cathode tubes. In particular, the cold-cathode tubes have such negative resistance characteristics that the resistance value thereof drops abruptly as the tube current thereof increases. The configuration of a cold-cathode tube lighting device is devised so as to conform to these electrical characteristics of the cold-cathode tubes. In particular, since thinning (downsizing) and electric power saving are emphasized for devices to which the cold-cathode tubes are applied, the cold-cathode tube lighting device is also strongly requested to be made smaller in size (particularly thinner) and lower in power consumption.
For example, as a cold-cathode tube lighting device according to a prior art, the device described below has been known (for example, see Patent documents 1 and 2). FIG. 14 is a circuit diagram showing a configuration of the cold-cathode tube lighting device according to the prior art. The cold-cathode tube lighting device according to the prior art includes a high-frequency oscillation circuit 100, a step-up transformer “T” and an impedance matching part 200.
The high-frequency oscillation circuit 100 converts a direct-current voltage supplied from a direct-current power source DC into an alternating-voltage having a high frequency, and applies the alternating-voltage to a primary winding L1 of the step-up transformer “T”. The step-up transformer “T” generates a voltage, which is extremely higher than a primary voltage, across both ends of a secondary winding L2 thereof. The high secondary voltage “V” is applied across both ends of a cold-cathode tube FL via the impedance matching part 200. For example, the impedance matching part 200 includes a series circuit of a choke coil “L” and a capacitor “C”. In this case, the capacitor “C” includes stray capacitances in the periphery of the cold-cathode tube FL. Impedance matching is performed between the step-up transformer “T” and the cold-cathode tube FL by adjusting the inductance of the choke coil “L” and the capacitance of the capacitor “C”.
During the time when the cold-cathode tube FL is off, when a voltage is applied to the primary winding L1 of the transformer “T”, a voltage VR across both ends of the cold-cathode tube FL is raised abruptly by a resonance of the choke coil “L” and the capacitor “C” of the impedance matching part 200, and the voltage VR exceeds a firing potential. As a result, the cold-cathode tube FL starts discharging and begins to emit light. Then, a resistance value of the cold-cathode tube FL drops abruptly as the tube current IR increases (negative resistance characteristics). Along with this drop in the resistance value of the cold-cathode tube FL, the voltage VR across both ends of the cold-cathode tube FL drops. At that time, the tube current IR is maintained stably by the action of the impedance matching part 200, regardless of the change in the voltage VR across both ends of the cold-cathode tube FL. Namely, the luminance of the cold-cathode tube FL is maintained stably.
In FIG. 14, the secondary winding L2 of the step-up transformer “T” and the choke coil “L” are shown as circuit elements different from each other. However, in an actual cold-cathode tube lighting device, a secondary winding of one leakage flux transformer was used for three purposes of step-up, choking and impedance matching. Accordingly, both the number of components and the size were reduced. Namely, in the cold-cathode tube lighting device according to the prior art, the leakage flux transformer was regarded as particularly advantageous in downsizing and thus used frequently.
Generally speaking, in the cold-cathode tube FL, the stray capacitance between the tube wall and the external grounding conductor (such as a case or reflecting plate of a liquid crystal display) is caused. For example, in such a configuration that one of the electrodes of the cold-cathode tube FL is grounded as in the cold-cathode tube lighting device disclosed in the patent document 1, only the electric potential of the other electrode fluctuates greatly with respect to the ground potential. Accordingly, when the stray capacitance between the tube wall and the external part is excessive, the leakage current flowing between the tube wall and the external increases excessively particularly near above-mentioned the other electrode. Particularly when the code cathode tube FL is long, the excessive increase of the leakage current may impair the uniformity of the tube current in the longitudinal direction. As a result, an imbalance in luminance may occur in the longitudinal direction of the cold-cathode tube FL.
In order to further raise the uniformity of the luminance in the longitudinal direction the cold-cathode tube, an intermediate point of the electrode potentials at both ends of the cold-cathode tube FL is preferably maintained at the ground potential. For example, with regards to the cold-cathode tube lighting device according to the prior art shown in FIG. 14, the secondary winding L2 of the step-up transformer “T” is grounded at a neutral point M2 thereof, and equivalent ballasts are connected to both ends of the cold-cathode tube FL, respectively (See patent document 2). By this configuration, the intermediate point of the electrode potentials at both ends is maintained at the ground potential. Namely, the electrode potentials at both ends are maintained asymmetrically with respect to the ground potential and the electrode potentials are fluctuated equally. Accordingly, in the cold-cathode tube FL, the distribution of the leakage current flowing between each part of the tube wall and the external is symmetrical with respect to the central part of the cold-cathode tube FL. Accordingly, in each cold-cathode tube, the imbalance in luminance in the longitudinal direction thereof is reduced, and this leads to the improved uniformly.
Further, when the intermediate point of the electrode potentials at both ends of the cold-cathode tube FL is maintained at the ground potential, the amplitude of the electrode potential with respect to the ground potential is halved while the amplitude of the voltage across both ends of the cold-cathode tube FL is maintained, unlike the case where the electrode at one end of the cold-cathode tube FL is grounded. Accordingly, since the leakage current is reduced, the imbalance of the distribution of the leakage current is reduced. Accordingly, the imbalance in luminance in the longitudinal direction of the cold-cathode tube FL is further reduced, and this leads to the further improved uniformly.
Patent document 1: Japanese patent laid-open publication No. 8-273862.
Patent document 2: Japanese patent laid-open publication No. 8-122776.